1. Field of the Invention
The present invention relates to an anomaly judgment apparatus for a secondary air supply system, which is adapted to supply secondary air to a portion of an exhaust path of an internal combustion engine located upstream of a catalytic converter disposed in the exhaust path.
2. Description of the Related Art
Conventionally, in order to eliminate harmful components (CO, HC, and NOx) from exhaust gas emitted from an internal combustion engine, a three-way catalytic converter (herein may be referred to merely as a “catalytic converter”) is disposed in an exhaust path of the internal combustion engine. When the temperature of the three-way catalytic converter is lower than an activation temperature needed for the three-way catalytic converter to exhibit its catalytic function, the three-way catalytic converter fails to sufficiently exhibit its exhaust purification function. Therefore, when the temperature of the three-way catalytic converter is lower than the activation temperature; for example, in the case where an internal combustion engine is started when the temperature of the internal combustion engine is near ambient temperature (outside air temperature) (hereinafter referred to as “at the time of cold start”), the temperature of the three-way catalytic converter must be increased as soon as possible so as to activate (warm up) the three-way catalytic converter.
In order to fulfill the above need, a secondary air supply system is widely used. The secondary air supply system supplies secondary air to a portion of the exhaust path located upstream of the three-way catalytic converter, whereby the three-way catalytic converter is actively warmed up by heat of reaction that is generated when oxygen contained in the secondary air oxidizes unburnt components (particularly HC) contained in exhaust gas.
Component parts of such a secondary air supply system generally include an air pump and an air switching valve. The air pump is adapted to introduce air into a secondary air supply path connected to a portion of the exhaust path located upstream of the three-way catalytic converter. The air switching valve is provided in a portion of the secondary air supply path located downstream of the air pump and adapted to control opening and closing of the secondary air supply path. When anomaly arises in such a component part of the secondary air supply system, the following problem arises. Even when an instruction to supply secondary air is issued to the secondary air supply system, secondary air is not supplied, and thus warming up of the three-way catalytic converter is delayed, resulting in an increase in the quantity of emissions. Alternatively, even when an instruction to stop supplying secondary air is issued to the secondary air supply system, secondary air continues being supplied, and thus the air-fuel ratio of exhaust gas shifts to the lean side, resulting in a drop in NOx elimination efficiency. Therefore, when anomaly arises in the secondary air supply system (component parts of the same), the occurrence of the anomaly must be detected.
In this connection, a secondary air supply system (anomaly judgment apparatus for the same) disclosed in Japanese Patent Application Laid-Open (kokai) No. 2003-83048 is configured as follows. A pressure sensor is provided to detect pressure in a portion of a secondary air supply path that extends between an air pump and an air switching valve, which are component parts of the secondary air supply system. Whether or not any of the component parts is anomalous is judged on the basis of pressure detected by the pressure sensor and the level of pulsation of the detected pressure (a value indicating (evaluating) the degree of pulsation; specifically, the amplitude of the detected pressure) stemming from exhaust pulsation.
The above disclosed secondary air supply system is configured such that air is introduced into the secondary air supply path from an intake path (specifically, from a portion of the intake path located downstream of an air filter and upstream of a throttle valve) of an internal combustion engine.
Also, intake air pulsation unavoidably occurs in a portion of the intake path located downstream of the throttle valve of the internal combustion engine. In a certain operation condition (which will be described later in detail) of the internal combustion engine, the intake air pulsation propagates into a portion of the intake path located upstream of the throttle valve. Thus, in the above disclosed secondary air supply system, in some cases, intake air pulsation may propagate into the secondary air supply path.
When intake air pulsation propagates into the secondary air supply path, the intake air pulsation influences an output of the pressure sensor. For example, in the case where the air switching valve is closed, and pulsation stemming from exhaust pulsation does not arise within the secondary air supply path (or the level of pulsation stemming from exhaust pulsation is low), the pulsation level of pressure detected by the pressure sensor must be calculated as being low, but in some cases the pulsation level is calculated as being high. Thus, the above disclosed anomaly judgment apparatus—which judges from the pulsation level whether or not the secondary air supply system (component parts of the same) is anomalous—may make a false judgment.